Protect STHelper from calls after detach

Now that we expose detach to upper layers, we should ensure that
calling a method on a detached STHelper strongly errors.

Fixes: 277390523
Bug: 272147641
Test: atest AlwaysOnHotwordDetectorTest#isSessionInvalid_afterDetach
Change-Id: I32cf30d711e27debed0d643a9627663830e2ccf6